An automatic analyzer which applies a light from a light source to a reaction solution obtained by mixing a biological sample, such as blood serum and urine, and a reagent, calculates the absorbance from the change of the amount of the transmitted light at a certain wavelength and determines the concentration of a substance measured according to the Lambert-Beer law is widely used (for example, PTL 1).
There are two types of reaction which are measured by an automatic analyzer, namely a color reaction mainly due to the reaction of a substrate and an enzyme and immune agglutination of an antigen and an antibody. An analysis using the former reaction is called a biochemical analysis and its examination items are enzymes, lipids, nitrogen compounds and the like. An analysis using the latter reaction is called an immunoassay and its examination items include microproteins, tumor markers, hormones, drugs in the blood and the like. Among the substances that are measured in the latter analysis, there are examination items in which detection with high sensitivity in a low-concentration range is required and examination items in which the quantitative value is important for the clinical diagnosis. For these items, latex immuno-nephelometry using latex particles with surfaces sensitized (bound) with an antibody as a sensitizer is used. In latex immuno-nephelometry, a light is applied to aggregates generated by the aggregation of the latex particles due to the substance to be measured, and the change of the amount of the light which has been transmitted without being scattered is measured. Because the change of the light amount after a certain period of time becomes larger as the concentration of the substance to be measured becomes higher, the concentration of the substance to be measured can be calculated from the change of the amount of the transmitted light.
Recently, it has been desired that the sensitivity of immunoassay be further improved. So far, it has been attempted to measure the scattered light, rather than the transmitted light, in an apparatus for performing latex immunoassay with higher sensitivity. Such alight scattering spectrophotometer is called a nephelometer and measures the intensity of the scattered light by applying a light to aggregates generated by the reaction of an antigen contained in a sample and an antibody contained in a reagent. For example, PTL 2 discloses an apparatus which separates the transmitted light and the scattered light using a diaphragm and measures the absorbance and the scattered light simultaneously.
In order to ensure the reliability and the adequacy of measurement results obtained using these automatic analyzers, various characteristics have been defined as objects for confirming the adequacy of a measurement method. For example, NPL 1 describes that limit of detection/limit of quantification is inspected in addition to the accuracy and the precision, as a characteristic which needs to be confirmed for the adequacy. A limit of detection is the lowest amount of a substance to be measured in a sample that can be detected, and a limit of quantification is the lowest amount of a substance to be measured at which the amount can be measured with a certain level of precision. A method for evaluating limit of detection/limit of quantification is described in NPL 2 for example.
In the present specification, a term “limit of detection/limit of quantification” is used for the sake of convenience. When described in this way, the term is used to mean at least one of a limit of detection and a limit of quantification and includes all the cases of a limit of detection only, a limit of quantification only, and a limit of detection and a limit of quantification.